(1) Field of the Invention
The present invention relates to a variable valve system for an internal combustion engine.
(2) Description of the Related Art
Conventionally, there has been known a variable valve system that includes a variable valve timing mechanism capable of changing valve overlap of intake and exhaust valves of an engine by varying the opening/closing timing of the intake and exhaust valves which are driven to open and close by valve mechanisms, and a control unit which controls the valve overlap of the intake and exhaust valves by operating the variable valve timing mechanism according to the operative state of the engine. The variable valve timing mechanism is comprised of a vane which is integrated with a cam shaft, and a housing which stores the vane in an oil chamber therein and is formed integrally with a sprocket for transmitting rotation of a crank shaft to the cam shaft. The variable valve timing mechanism changes the amount of oil, which is supplied into the oil chamber, according to the operative state of the engine to shift the phase of a cam to the advanced angle side/the retarded angle side to vary the valve opening/closing timing.
If the phase of an exhaust cam is on the retarded angle side, so-called internal EGR (Exhaust Gar Recirculation) amount is increased due to a large valve overlap of the intake and exhaust valves. On the other hand, if the phase of the exhaust cam is on the advanced angle side, the internal EGR amount is decreased due to a small valve overlap of the intake and exhaust valves. When the engine is revolving at an extremely low speed, e.g., when the engine is started, an increase in internal EGR amount decreases the quantity of fresh air, while a decrease in the internal EGR amount increases the quantity of fresh air. In contrast with the exhaust cam, if the phase of an intake cam is on the retarded angle side, the valve overlap of the intake and exhaust valves is small, and if the phase of the exhaust cam is on the advanced angle side, the valve overlap of the intake and exhaust valves is large. Valve springs apply force against each of the intake and exhaust cams. The pressure of oil, which is supplied into the oil chamber in the variable valve timing mechanism, is low immediately after starting or during idling. For this reason, immediately after staring or during idling, rotation of a crank shaft is transmitted to the sprocket, and the sprocket tries to rotate, but each cam is forced in such a direction as to inhibit rotation thereof, and hence the phase of each cam is relatively brought into the retarded angle side.
Therefore, when the engine is started, if the phase of the exhaust cam is brought into the retarded angle side, it affects the startability. Therefore, it has been proposed that the variable valve timing mechanism is provided with a lock pin which locks the phase of the cam in the most advanced angle side. The lock pin is actuated by hydraulic control when the phase of the cam is brought into the most advanced angle side, and is configured to hold the exhaust cam in the most advanced angle position. Thus, if the exhaust cam is brought into and held in the most advanced angle position when the engine is idling immediately before being stopped, the engine can be stopped with the exhaust cam being held in the most advanced angle position.
According to the above described arrangement in which the lock pin actuated by hydraulic control holds the exhaust cam on the most advanced angle side, there may be a case where the exhaust cam cannot be held in the most advanced angle side by the lock pin. The driver usually stops the engine by turning off an ignition key when the engine is idling. In the case where a vehicle is equipped with an automatic transmission, the engine is stopped when a shift lever is positioned in either one of a P-range (parking range) and an N-range (neutral range), which mean that the vehicle is at a standstill. If the engine is idling while the shift lever is positioned in either one of the P-range and the N-range, the phase of the exhaust cam is brought into the retarded angle side, and therefore, if the ignition key is turned off in this state, the exhaust cam cannot be locked on the most advanced angle side. This deteriorates the startability when the engine is restarted, and causes unstable idling after the engine is started.